As is known, the decay of the building materials, in particular, marble, sandstones, tiles, and of the artistic and decorative articles manufactured from such materials, is caused by two different kinds of phenomena: chemical phenomena and physical phenomena, both occurring in the presence of water.
A physical phenomenon, which acts on all the building materials in the one which is caused by succession of frost and thaw of water absorbed into the material pores. This leads to variation of specific volume and to tension, which often cause fractures and/or loss of cohesion or in any case a separation among the individual particles forming the material.
Another physical phenomenon is the dissolution process in water of limestone, which is contained as an essential component in the marbles and as a binder in the stones, with consequent loss of cohesion of the granules.
Equally important for the decay of the materials and manufactured articles exposed to the open atmosphere is the aggregate of chemical phenomena. This phenomena substantially consists in the aggression of the carbonated parts of the material or of the manufactured article by the pollutants which are present in the air (for example carbon dioxide, sulphur dioxide, nitrogen oxide). The pollutants react with the material in the presence of water. Their concentration is particularly high when rain begins.
It is known that to eliminate or at least to sensibly reduce the above-mentioned drawbacks, it is necessary to remarkably reduce the penetration rate of water inside the material to be protected.
To achieve this object, chemical compounds, which can possess a twofold property, i.e., the hydrophobic and consolidating property, are usually applied onto the outer surface of the material to be protected.
To exert the desired action and not to cause, at the same time, collateral decay phenomena or undesired side effects, the consolidating/protective compound must possess the following essential characteristics:
1 Restoring ability of the cohesion between the particles of the outermost layer of the decayed material and the underlying integral part.
2. Exhibiting an adequate mechanical strength of the so consolidated outermost layer.
3. Retaining, in the consolidated outermost layer, characteristics of elasticity to prevent the formation, as occurs with other products, of a stiff surface layer having mechanical characteristics which are different from those of the substrate.
4. Chemical inertia towards the material to be protected/consolidated.
5. Low volatility.
6. Stability to the action of atmospheric agents, to sunlight and to heat.
7. Not altering the material chromaticity.
8. Retaining for a long time its own solubility characteristics to secure treatment reversibility.
As is known, for the consolidation and protection of stony material, film-forming substances, such as glues, drying oils, resins and polymeric matters are often used. References on the use of such substances are contained in the book by Amoroso and Fassina "Stone Decay and Conservation", Elsewier Ed., Amsterdam (1983).
Among such substances, which are all not thoroughly satisfactory for the above purposes, the most commonly used are the following: polyethylene glycols and their methylethers, which exhibit, however, a high solubility in water, wherefore the use thereof must be limited to the protection of articles not exposed to the environment; polyvinyl acetate and polyvinyl alcohol, which exhibit the drawback of being sensible to water, in which the former, moreover, hydrolyzes thereby releasing acetic acid; polymethylmethacrylate and acrylic resins in general, which, however, tend to yellow in the course of time and are not capable of providing a complete protection from SO.sub.2 contained in the atmosphere, while the thermosetting acrylic copolymer, endowed with better characteristics, exhibits the drawback of being irreversible; silicone resins, which, however, besides the possibility of releasing methanol, are affected by the drawback of cross-linking with formation of a product, which cannot be removed; epoxy reins, which tend to yellow and, because of their having the structure of thermosetting resins, make it difficult or impossible to carry out reversible treatments; polyester resins, which have the drawback of being sensible to water (owing to which they tend to saponify) and also of cross-linking, wherefore they lose the reversibility characteristic.
Such products, which are utilized in the treatment of the stony and cement articles, besides not exhibiting in general, as explained above, characteristics of stability to chemical and physical attacks, drastically reduce the permeability of the treated surface to air and to water vapor. As a consequence thereof, extremely remarked alteration phenomena occur in the manufactured articles when exposed to the environment underneath the impermeable film, owing to condensation water inside the materials.
The use, for the above purposes, of perfluoropolyethers applied in the form of solution is chlorofluorocarbons, as is described in European patent application Nos. 59,100 and 192,493, or in the form of aqueous microemulsions, as is described in European patent application No. 89/106,150, has solved the problem concerning the protection. However such compounds, because they are liquids, cannot exert any consolidating action.
The use of vinylidene fluoride copolymers as is described in European patent application No. 192,494 has solved the problem concerning the consolidation, but it involves the use of considerable amounts of inflammable solvent.
The above references are hereby incorporated by reference.